In a superconducting accelerator system, a charged particle beam is directed into an accelerating cavity, and radio frequency electromagnetic waves are introduced via an input coupler. A charged particle in the cavity is accelerated by a radio frequency electric field generated in the cavity. The input coupler introduces into the cavity radio frequency waves generated at a radio frequency generator (e.g., a klystron) and propagated by a waveguide.
There are two types of input couplers: a coaxial coupler; and a rectangular waveguide coupler. PTL 1 discloses an input coupler including a hollow connecting part that continues from an open end of a hollow rectangular part to a cylindrical flange part to integrally connect them. Accordingly, in the invention disclosed in PTL 1, both of the flange part of the input coupler and a flange part of a waveguide are circular, thereby applying a uniform load to a seal member sandwiched by the flange parts. As a result, the sealability is enhanced.